Alloy steels



Dec. 8, 19

Filed Dec. 4, 1956 T. H. MIDDLEHAM ALLOY STEELS 5 Sheets-Sheet 1 FIG. I.

LUg'HNIUM l0 4 Inventors Thomas Harold Middle 11h Eric Win earl (Ia/beck B Q 2 I M Attorney T. H. MlDDLEHAM ETAL ALLOY STEELS Dec. 8, 1959 Filed Dec. 4, 1956 ALUMINIUM 10 5 Sheets-Sheet 2 lmwnlnr Thorn D5 H ruhl Mio ouela'a m Eric Wih earls Colbev-f/ f Attorney United States Patent() ALLOY STEELS Application December 4, 1956, Serial No. 626,147

Claims priority, application Great Britain December 5, 1955 10 Claims. (Cl. 75-124) This invention relates to neutron arresting materials.

Boron is known to be such a material and has been used as such in control rods for atomic reactors. For that purpose, it is highly desirable that the boron be in the form of widely dispersed particles of small size, the neutron arresting area then being very substantial. Boron being a metal, it has the advantage that it can be incorporated in an alloy, the remaining constituents of the alloy forming a carrier for the boron.

Boron is sometimes added to low carbon steels to improve their hardenability. Only small amounts can, however, be tolerated. For example, boron in excess of 0.007% in a low carbon steel will induce such hot shortness as to make it extremely difficult to forge, roll, press, extrude or draw the steel or subject it to any other hot working. These amounts of boron are much too small to be of interest in atomic reactors.

We have, however, made control rods consisting of alloy steels containing up to 2% boron. As control rods, they have proved successful in use but the fashioning of the rods has been attended by the greatest difficulties and extreme care has to be given not only to the manufacturing processes but also to the subsequent handling of the control rods. These considerations led us to the conclusion that 2% boron must be considered to be the absolute practical limita limitation which, of course, puts a limitation on the life of the control rod.

We have now discovered how to obtain an alloy suitable for use in the manufacture of control rods which contains substantially more than 2% boron.

According to the invention, the deterioration in workability resulting from the inclusion of boron in the alloy is compensated at least to some extent by the inclusion in the alloy of aluminium.

The alloys which come into question are alloy steels which do not contain any ingredient or more than the permissible quantity of any ingredient which is unacceptable in an atomic reactor because of the irradiation characteristics of the ingredient. For the purpose of the present application, by steel is meant any ferrous alloy containing up to 0.25% carbon and at least 70% of iron. The exact composition of the steel is not of primary importance. The steel, being merely a carrier for the boron, does not need to have well-defined mechanical characteristics such as tensile strength, ductility, resistance to creep, hardness and so on nor other physical characteristics such as resistance to scaling, corrosion and so on, as long as it can be forged or subjected to any other hot working process for the manufacture from it of a rod or tube suitable for use in an atomic reactor. Where, in what follows, reference is made to workability, it will be understood that the term includes all such processes.

We have succeeded in producing suitable alloy steels containing up to about 4% boron by the above-mentioned expedient of the inclusion :in the alloy of controlled amounts of aluminium. The result of our re- "ice searches is shown diagrammatically in Figures 1 and 2 in the accompanying drawings.

Figure 1 consists of a graph the vertical ordinates of which indicate percentage aluminium content and the horizontal ordinates percentage boron content. The curve indicates the limiting values of aluminium content for any particular boron content and has been obtained from the results of a very large number of tests made with specimens of steels of a composition which, but for their boron and aluminium contents, would classify as mild steels.

Figure 2 is an enlargement of part of the curve of Figure 1 and indicates the lower limiting values of the aluminium range.

It will be seen from the graph that with a boron content of 2%, the aluminium content must lie between 0.08 and 13%; with a boron content of 3%, the aluminium content must be between 0.26 and 10.3%; with a boron content of 4%, the aluminium content must lie between 0.45 and 7.2%. The range of aluminium content becomes smaller as the boron content increases until, with a boron content of about 4% the range becomes zero.

We are not able to offer any explanation of the phenomenon whereby the inclusion of controlled amounts of aluminium-a metal the workability of which is low in comparison with most other metals-compensates at least to some extent the loss of workability resulting from the presence of boron in the alloy. The choice of aluminium as an improver of workability is one which could not have been made on the basis of any known metallurgical theory.

It is to be understood that the alloys in accordance with the invention are not necessarily easily workable. We have, however, satisfied ourselves by numerous experiments that they can be forged or otherwise worked, with reasonable care. In many cases, it is desirable, if not essential, that the temperature be kept within fairly close limits and that other factors known'to metallurgists as being of importance, such, for example, as weight of blow, rate of reduction of cross-sectional area and so on, be similarly controlled. We have classified as unworkable all those compositions which we have found to be unforgeable when taking all the precautions which a steelworker would take who wished to achieve success.

We have successfully forged all the alloys shown in the table which appears further on herein at a maximum temperature 1080 C. We have found the bestforging technique to consist in light hammer blows with a slow reduction in cross-sectional area.

Our researches lead us to believe that the respective proportions of aluminium and boron indicated in the graph shown in the drawing are valid for steels of widely differing compositions. As already indicated, it appears that the carbon content should not exceed 0.25% and the iron content should not be below 70%.

In general, the higher the carbon content, the less workable does the alloy become. If a carbon content of 0.25 is exceeded, even the addition of aluminium, which is the point of this invention, does not make the alloy sufficiently easily workable for it to be acceptable.

For boron contents of 24%, the lower limiting value of the aluminium content can be calculated approximately from the formula Al=0.182B0.28

in which Al is the percentage aluminium content and B is the percentage boron content.

For boron contents of 44%% the lower limiting value of the aluminium content can be calculated approximately from the formula The upper limiting value can be similarly calculated from the formula Al =60(4.82B), or Al= /60(4.825 While it appears that the alloy steels may contain any -of the ingredients usually incorporated in steels, it is naturally advisable to choose a composition which does not include ingredients (other than aluminium and boron) which are known or are suspected to produce poor workability. For example, any substantial proportion of titanium, sulphur, phosphorus and so on should be avoided. Bearing in mind that a neutron arresting alloy will not generally be required to any well-defined physical characteristics other than a reasonable degree of workability, the exclusion of such elements should not .prove to be disadvantageous.

The ingredients other than iron, carbon, boron and aluminium must besuch or be present in amounts such as to be acceptable in an atomic reactor. In this connection,

The manganese content should not exceed 0.76% The chromium content should not exceed 0.75% The cobalt content should not exceed 0.01%

The nickel content should not exceed 0.10%

The silicon content should not exceed 0.70% The sulphur content should not exceed 0.04% The phosphorus content should not exceed 0.04% The copper content should not exceed 0.25%

The titanium content should not exceed 0.40% The molybdenum content should not exceed trace The tungsten content should not exceed trace The limits given above are determined by the irradiation characteristics of the elements and the restriction involved, in some-cases, adds to the difiiculty of producing a workable alloy. For example, greater amounts than those indicated above of manganese, nickel, cobalt or copper, which enhance workability are ruled out because of irradiation ditficulties.

As indicated above, the needs of industry can be met by the inclusion of boron as a neutron arrester in mild steels-i.e. in steels of the CSiMn type. Some compositions which'have been found satisfactory and which are among those on which the graph shown in the drawmg has been based are-given 1n the following table:

Composition Alloy Carbon Silicon Manganese Boron Aluminium Tests made on a large number .of different alloys appear to show thatathe boron and aluminium contents do not affect such mechanical properties as the ultimate tensile strength to any extent which is material to the caps 14. Caps 14 are also the means by which the control rod is connected to the control mechanism. Each cap is bored so as to allowthe passage through the tube 10 of the coolant gas.

Control rods which we have successfully manufactured are approximately 20 feet long and have boron steel tubes varying from 1 /2 to 3 inches in diameter.

We claim:

1. An alloy steel comprising carbon in an amount not exceeding 0.25%, at least iron, and amounts of boron and aluminium in the relative proportions indicated by and contained within the area defined by the curve in the graph shown in the accompanying drawing designated Fig. 1 for percentages of boron above 2%.

2. An alloy steel having essentially the characteristics of mild steel and comprising carbon in an amount not exceeding 0.25 and amounts of boron and aluminum substantially in the relative proportions indicated by and contained within the area defined by the curve in the graph shown in the accompanying drawing designated Fig. 1 for percentages of boron above 2%.

3. A ferrous base alloy comprising at least 70% iron, carbon in an amount not exceeding 0.25%, from 2 to 4%% boron and sufiicient aluminum Within the range 0.08 to 13% to impart workability to the alloy, said alloy being further characterized by the exclusion therefrom of objectionable amounts of elements tending to impart irradiation characteristics to the alloy.

4. A ferrous base alloy comprising carbon in an amount not exceeding 0.25 from 2 to 4% boron, and a percentage aluminum content not less than 0.1823

0.28 nor more than /60(4.82B), B being the percentage boron content.

5. A ferrous base alloy comprising carbon in an amount not exceeding 0.25%, from 4 to 4 4% boron, and a percentage aluminum content not less than 2.53

B 20B+39.8 nor more than /60(4.82B), B being the percentage boron content.

6. An alloy steel comprising carbon in an amount not exceeding 0.25 at least 2% to about 4% of boron and suflicient aluminum within the range 0.08 to 13% to impart workability to the alloy, said alloy being further characterized by the exclusion therefrom of objectionable amounts of elements tending to impart irradiation characteristics to the alloy.

7. An alloy steel comprising carbon in an amount not exceeding 0.25%, approximately 2% boron and from 0.08 to 13% aluminum.

8. An alloy steel comprising carbon in an amount not exceeding 0.25%, approximately 3% boron, and from 0.26 to 10.3% aluminum.

9. An alloy steel comprising carbon in anamount not exceeding 0.25 approximately 4% boron and 0.45 to 7.2% aluminum.

10. A control rod for an atomic reactor made with a ferrous base alloy comprising carbon in an amount not exceeding 0.25%, at least 70% iron, from 2 to 4%% boron and sutficient aluminum within the range 0.08 to 13% to impart workability to the alloy, said alloy being further characterized by the exclusion therefrom of 0bjectionable amounts of elements tending to impart irradiation characteristics to the alloy.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ALLOY STEEL COMPRISING CARBON IN AN AMOUNT NOT EXCEEDING 0.25%, AT LEAST 70% IRON, AND AMOUNT OF BORON AND ALUMINUM IN THE RELATIVE PROPORTIONS INDICATED BY AND CONTAINED WITHIN THE AREA DEFINED BY THE CURVE 